Technical Field
The present invention relates to an image forming apparatus such as a copying machine or a printer that uses an optical scanner including a rotary drive source and a rotary polygon mirror to execute scanning of a laser beam, and an image forming control program.
Description of the Related Art
An image forming apparatus is known that executes image formation of a predetermined number of lines in a main scanning direction with a light beam according to image data, and repeats image formation of the light beam for each predetermined number of lines, in the main scanning direction in a sub-scanning direction, to execute image formation for one page.
As an example, in an electrophotographic image forming apparatus, an optical scanner including a rotary drive source (polygon motor) and a rotary polygon mirror (polygon mirror) is used, and scanning is executed of a laser beam emitted in accordance with image data, in the main scanning direction, and in parallel with this, an image is formed by the laser beam on an image carrier (photosensitive member) rotating in the sub-scanning direction. In this case, the laser beam is emitted in accordance with the image data, using a clock signal (write clock) called a dot clock as a reference.
Due to a slight rotation irregularity of the polygon motor and a minute reflection surface accuracy error of the polygon mirror, jitter (fluctuation in the time axis direction) is generated in the laser beam scanning the image carrier, and a so-called short period jitter phenomenon occurs. Hereinafter, the short period jitter is simply referred to as jitter.
Due to the jitter, a dot position shift in the main scanning direction is periodically generated for each reflection surface of the polygon mirror (see FIG. 11), and due to interference with a screen image, the dot position shift becomes easy to be visually recognized as image quality degradation such as a horizontal stripe (see FIG. 12).
FIG. 11 schematically illustrates the shift in the main scanning direction of each reflection surface of six-sided rotary polygon mirror. In this case, as illustrated in FIG. 11, main scanning direction ends formed on the image carrier by the reflection surfaces #1 to #6 of the rotary polygon mirror are at different positions, respectively. When image formation of an oblique line is executed by such a dot group having variation, image quality degradation occurs, such as cyclic fluctuation as illustrated in FIG. 12.
To suppress image quality degradation, for example, various means are described in JP 2002-267961 A and JP 2003-140068 A.
To realize high image quality with such an image forming apparatus, it is important to align main scanning direction start positions and main scanning direction end positions of laser beams, that is, to uniform main scanning lengths between the laser beams to eliminate the shift in the main scanning direction.
In JP 2002-267961 A, a frequency of a write clock is adjusted for each surface of a polygon mirror with a Start Of Scan (SOS) signal on the start position side in the main scanning direction and an End Of Scan (EOS) signal on the end position side in the main scanning direction, whereby the main scanning length is controlled to be constant. Accordingly, by aligning the main scanning direction start positions and end positions, an error is eliminated in a part corresponding to the outer frame of the image. However, as illustrated in FIG. 13, an error due to flatness of the polygon mirror reflection surface remains in a part between the start position and the end position in the main scanning direction.
In JP 2003-140068 A, jitter information is stored in advance in a plurality of positions in the main scanning direction for each surface of the polygon mirror, a correction characteristic approximated by a straight line is obtained for the dot position shift at a plurality of positions generated in accordance with jitter on each surface, and the frequency and phase of the write clock are adjusted in accordance with the correction characteristic. In FIG. 14, the slope of the correction characteristic corresponds to the frequency of the write clock, and the intercept of the correction characteristic (the value of the vertical axis at the position of the horizontal axis 0) corresponds to the phase of the write clock. In this case, it has been thought that relatively satisfactory correction is possible, but it has been found that it is not possible to cope with a change with time of the jitter.
For example, it has been found that, in a case of a polygon mirror that continues to rotate at high speed, a change with time occurs in the flatness of the reflection surface due to difference in centrifugal force acting at each position of the reflection surface. In this way, it has become clear that the change with time of the jitter occurs; however, it has been found that the method of JP 2003-140068 A cannot cope with the change with time.